In the lithography technology, an exposure tool for manufacturing an integrated circuit by transferring a minute circuit pattern onto a wafer has hitherto been widely utilized. With the trend toward a higher degree of integration and a higher function of an integrated circuit, the refinement of the integrated circuit is advancing. The exposure tool is hence required to form a circuit pattern image with high resolution on a wafer surface at a long focal depth, and shortening of the wavelength of an exposure light source is being advanced. The exposure light source advances from the conventional g-line (wavelength: 436 nm), i-line (wavelength: 365 nm) or KrF excimer laser (wavelength: 248 nm), and an ArF excimer laser (wavelength: 193 nm) is coming to be employed. Also, in order to cope with a next-generation integrated circuit whose circuit line width will become not more than 70 nm, an immersion lithography technique and a double exposure technique, each using an ArF excimer laser, are regarded as being leading. However, it is considered that even these techniques would be able to cover only the generation with a line width of up to 45 nm.
Under such a technical trend, a lithography technique using EUV light as a next-generation exposure light source (EUVL) is considered to be applicable over the 32-nm and subsequent generations and is attracting attention. EUV light indicates light with a wavelength band in a soft X-ray region or a vacuum ultraviolet region, and this is specifically light at a wavelength of approximately from 0.2 to 100 nm. At present, studies are being made on light at 13.5 nm as the lithography light source. The exposure principle of the EUV lithography is the same as the conventional lithography in that a mask pattern is transferred using a projection optical system. However, since there is no material capable of transmitting light therethrough in the EUV light energy region, a refractive optical system cannot be used. Accordingly, the optical systems are all reflecting optical systems.
The reflecting optical system for use in EUVL includes, for example, a reflective type mask (hereinafter, in the context of the present invention, referred to as a “mask for EUVL”) and a mirror such as light collection optical system mirror, illumination optical system mirror and projection optical system mirror (hereinafter, in the context of the present invention, referred to as a “mirror for EUVL”).
The mask blank for EUVL used in the production of a mask for EUVL is basically composed of (1) an optical component for EUVL (for example, a glass substrate), (2) a reflective multilayer film formed on the optical surface of the optical component for EUVL and (3) an absorber layer formed on the reflective multilayer film. On the other hand, the mirror for EUVL is basically composed of (1) an optical component for EUVL (for example, a glass substrate) and (2) a reflective multilayer film formed on the optical surface of the optical component for EUVL.
As for the optical component for EUVL, a material having a low thermal expansion coefficient not causing a strain even under irradiation of EUV light is required and use of a glass having a low thermal expansion coefficient or a glass-ceramics having a low thermal expansion coefficient is being studied. In the following, in the context of the present invention, the glass having a low thermal expansion coefficient and the glass-ceramics having a low thermal expansion coefficient are collectively referred to as a “low expansion glass” or an “extremely low expansion glass”.
As for such a low expansion glass and an extremely low expansion glass, a silica glass having added thereto a dopant for decreasing the thermal expansion coefficient of the glass is being most widely used. In addition, the dopant added for the purpose of decreasing the thermal expansion coefficient of the glass is typically TiO2. Specific examples of the silica glass where TiO2 is added as a dopant include ULE (registered trademark) Code 7972 (produced by Corning) and Product No. AZ6025 produced by Asahi Glass Co., Ltd.
As for the reflective multilayer film, a film having a structure where a plurality of materials differing in the refractive index in the wavelength region of EUV light as the exposure light are periodically laminated in the nm order is used, and this is most commonly a reflective multilayer film formed by alternately laminating a molybdenum (Mo) layer as a layer having a high refractive index in the wavelength region of EUV light (high refractive index layer) and a silicon (Si) layer as a layer having a low refractive index in the wavelength region of EUV light (low refractive index layer) and thereby enhanced in the light reflectance when irradiating the layer surface with EUV light. For the absorber layer, a material having a high absorption coefficient for EUV light, specifically, a material comprising, for example, Cr or Ta as a main component, is used.
If minute irregularities are present on the optical surface of an optical component for EUVL, this adversely affects the reflective multilayer and absorber layer formed on the optical surface. For example, if minute irregularities are present on the optical surface, the periodic structure of the reflective multilayer film formed on the optical surface is disturbed and when EUVL is performed using a mask or mirror for EUVL produced using the optical component for EUVL, the desired pattern may be partially lost or an extra pattern other than the desired pattern may be formed. The disorder of the periodic structure of the reflective multilayer film, which is ascribable to irregularities present on the optical surface, is a serious problem called a phase defect and irregularities larger than a predetermined size are preferably not present on the optical surface.
Non-Patent Documents 1 and 2 describe requirements regarding the defects of a mask for EUVL and a mask blank for EUVL, and these requirements regarding the defect are very severe. In Non-Patent Document 1, it is indicated that the presence of a defect exceeding 50 nm on a substrate causes a disorder in the structure of the reflective multilayer film, giving rise to an unexpected profile of the pattern projected on the resist on an Si wafer, and is not allowed. Also, in Non-Patent document 1, it is indicated that for preventing an increase in the line edge roughness of a pattern projected on the resist on an Si wafer, the surface roughness of the substrate must be less than 0.15 nm in terms of RMS (root-mean-square roughness). In Patent Document 2, it is indicated that on a mask blank coated with a reflective multilayer film, which is used for EUV lithography, the presence of a defect exceeding 25 nm is not allowed.
Also, Non-Patent Document 3 describes what size of a defect on a substrate is likely to be transferred. Non-Patent Document 3 indicates that a phase defect may change the line width of the printed image. More specifically, it is taught that a phase defect having a surface bump with a height of 2 nm and FWHM (full width at half maximum) of 60 nm is a size marking the border as to whether the defect is likely to be transferred or not and a phase defect in this size causes an unallowable change of 20% (on a mask, 140 nm) in the line width for a line of 35 nm.
Furthermore, Patent Document 1 describes a method for repairing the concave-convex portion by collecting laser light in the vicinity of the surface or back surface of the concave-convex portion of a mask substrate for EUVL.    Non-Patent Document 1: “Specification for extreme ultraviolet lithography mask blank”, SEMI, pp. 37-1102 (2002)    Non-Patent Document 2: “Specification for absorbing film stacks and multilayers on extreme ultraviolet lithography mask blanks”, SEMI, pp. 38-1102 (2002)    Non-Patent Document 3: Alan Stivers, et al., “Evaluation of the Capability of a Multibeam Confocal Inspection System for Inspection of EUVL Mask Blanks”, SPIE, Vol. 4889, pp. 408-417 (2002)    Patent Document 1: JP-A-2008-027992